Missing return statement in Java stream

I have the following list and map:

final List<Product> productList = productService.findAll(uuidList);

Map<UUID, Product> productMap = new HashMap<>();

I want to add the element pairs to the map as shown below:

productList.stream().map(product -> {
    productMap.put(product.getUuid(), product);
});

However, the last line throws "Missing return statement" error. When I update as shown below, it has gone, but this time return is useless. So, how should I add UUID and Product pair to my map?

>Solution :

No need to use map here, you can simply use forEach. The map method is intended to create another stream whose elements are "mapped" from the original stream.forEach just applies a consumer on each element, in this case the consumer is putting the element into a hash map.

productList.stream().forEach(product -> 
    productMap.put(product.getUuid(), product)
);

You can also simplify the code by using a map collector:

Map<UUID, Product> productMap = productList.stream()
                                           .collect(Collectors.toMap(Product::getUuid, Function.identity()));

As for the reason why the original code with map is not compiling, it’s because the map method expects a Function which by definition takes a value and returns something. Since you’re using curly brackets, you should add a return statement. Alternatively you can remove the brackets, but keep in mind what I mentioned above, and that the return value is meaningless in this case since it’s not used:

productList.stream().map(product -> 
    productMap.put(product.getUuid(), product);
);